Physical aspects and modelling of turbulent MILD combustion
Moderate or Intense Low-oxygen Dilution (MILD) combustion is one of combustion technologies which can improve efficiency and reduce emissions simultaneously. This combustion type is characterised by the highly preheated reactant temperature and the relatively small temperature rise during combustion due to the intense dilution of the reactant mixture. These unique combustion conditions give MILD combustion very attractive features such as high combustion efficiency, reduction of pollutant emissions, attenuation of combustion instabilities and flexibility of the flow field. However, our understanding of MILD combustion is not enough to employ the MILD combustion technology further for modern combustion devices.
In this thesis, Direct Numerical Simulation (DNS) has been carried out for turbulent MILD combustion under four MILD and classical premixed conditions. A two-phase strategy is employed in the DNS to include the effect of imperfect mixing between fresh and exhaust gases before intense chemical reactions start. In the simulated instantaneous MILD reaction rate fields, both thin and distributed reaction zones are observed. Thin reaction zones having flamelet like characteristics propagate until colliding with other thin reaction zones to produce distributed reaction zones. Also, the effect of such interacting reaction zones on scalar gradient has to be taken into account in flamelet approaches.
Morphological features of MILD reaction zones are investigated by employing Minkowski functionals and shapefinders. Although a few local reaction zones are classified as thin shape, the majority of local reaction zones have pancake or tube-like shapes. The representative scales computed by the shapefinders also show a typical volume where intense reactions appear.
Given high temperature and existence of radicals in the diluted reactants, both reaction dominated and flame-propagation dominated regions are locally observed. These two phenomena are closely entangled under a high dilution condition. The favourable conditions for these phenomena are investigated by focusing on scalar fluxes and reaction rate.
A conditional Probability Density Function (PDF) is proposed to investigate flamelet/non-flamelet characteristics of MILD combustion. The PDF can be obtained by both numerically and experimentally. The PDF shows that MILD combustion still has the direct relationship between reaction rate and scalar gradient, although the tendency is statistically weak due to the distributed nature of MILD reaction zones.
Finally, based on the physical aspects of MILD combustion explained in this work, a representative model reactor for MILD combustion is developed. The model reactor is also used in conjunction with the presumed PDF for a mean and filtered reaction rate closure. The results show a good agreement between the modelled reaction rate and the DNS results.